A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits radiation energy in another portion of the electromagnetic spectrum. One important class of phosphors includes crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. The color of the emitted radiation can be controlled in part by the several combinations of activator and inorganic compound. Most useful and well-known phosphors emit radiation (also referred to as light herein) in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range. The phosphors may be used in a light emitting diode (LED), for example, to generate colored emissions that may generally not be obtained from the LED itself.
Light emitting diodes (LEDs) are semiconductor light emitters often used as a replacement for other light sources, such as incandescent lamps. A combination of LED-generated light and phosphor generated light may be used to produce white light. The most popular white LEDs are based on blue-emitting GaInN chips. The blue-emitting LEDs are coated with a phosphor or a phosphor blend including red-emitting, green-emitting and blue-emitting phosphors that converts some of the blue radiation to a complementary color, for example a yellow-green emission. The combined light from the phosphor and the LED chip provides white light having a color point with corresponding color coordinates (ccx and ccy) and correlated color temperature (CCT), and its spectral distribution provides a color rendering capability, measured by the color rendering index (CRI).
Generation of “white light” is currently achieved by so called “white LEDs” that are constituted by employing a blue LED in conjunction with a yellow-green-emitting, cerium-doped yttrium aluminum garnet known as “YAG,” having the formula Y3Al5O12:Ce3+. YAG has been historically used in these lighting systems because of its high quantum efficiency under blue light excitation and a broad emission spectrum that peaks in the yellow spectral region. One drawback of YAG-based lighting systems is their relatively poor color rendering properties and high color temperature. For example, when an object is illuminated under such currently used white LEDs, the object colors appear somewhat different from the colors produced by natural light.
A phosphor composition used for LED is desired to efficiently absorb blue radiation, emit with high quantum efficiency, and be stable in light output at elevated temperatures. Although numerous phosphors have been proposed in the past several years, the range of phosphors suitable for LEDs is limited. Therefore, there is a need for phosphors that produce improved color rendering in white-light-emitting, solid-state lighting systems.